High quality factor (Q) resonators are widely used in wireless power harvesting and wireless power transfer systems. However, the operating bandwidth of high Q resonators is very limited since the bandwidth of linear resonators is inversely proportional to Q (BW=f0/Q, where f0 is the resonance frequency). Although high Q resonators allow for efficient operation, their narrow bandwidth reduces the systems' tolerance to coupling factor variation, and to frequency detuning due to environmental effects, coupling to nearby objects, component aging and tolerances.
Specifically, high Q resonators are often used in wireless power transfer (WPT) systems, and wireless power harvesters which operate at low level RF power levels. In WPT systems, high Q resonators are often used to achieve high transfer efficiency at longer distances. Such systems exhibit “frequency split” phenomenon. The highest performance is achieved when the resonant coils are critically coupled to each other, corresponding to a specific coupling factor and operation frequency. At coupling factors beyond the critical coupling (i.e. over-coupled regions, e.g. at closer distances), frequency split phenomenon occurs meaning that the maximum power transfer efficiency is achieved at two different frequencies apart from the original operation frequency. At coupling factors below the critical coupling (i.e. under-coupled region, for example when the distance between the two coils is increased), the optimum frequency of operation remains the same, yet the transfer efficiency decays exponentially. In practical applications, a WPT system is usually designed to operate at the critical-coupled condition to achieve the best power transfer efficiency. Therefore, its performance is susceptible to coupling factor variation. Coupling factor variation can also occur due to misalignment between the coils, as well as coupling to nearby objects.
In wireless power harvesters, the RF power collected by an antenna or any energy pickup device is rectified and regulated before being delivered to a load. Since power harvesters often operate at low power levels, their sensitivity is one of the key design considerations. A high quality factor (Q) resonator placed in the impedance matching network in-between the antenna and the rectifier increases the RF voltage through Q multiplication, which is desirable for overcoming the threshold voltage of the rectifiers and therefore improving the rectification efficiency at low power levels. However as mentioned before, harvesters employing conventional high Q resonators are vulnerable to frequency misalignment and frequency drift. Although it is possible to design an active frequency-tracking mechanism for these systems, such a tracking circuit not only requires power for its operation, but also increases complexity and cost.
This disclosure presents a new approach for improving the bandwidth of high Q RF resonators. A nonlinear resonator is developed to enhance the bandwidth while maintaining high resonance amplitude.
This section provides background information related to the present disclosure which is not necessarily prior art.